Hollow, multi-layered, cross-linked plastic structures and process for producing same

ABSTRACT

Hollow, multi-layered plastic structures are provided comprising a layer of thermoplastic polyester material and a layer of polyethylene, the polyethylene and polyester layers being crosslinked together at their interface. A process for producing such structures which comprises charging a mold with thermoplastic polyester material, rotating and heating the mold so as to fuse and form a layer of the polyester material on the inner surface of the mold, cooling the mold to a point below the melting point of the polyester but above the melting point of a subsequently added cross-linkable polyethylene material, stopping the rotation and charging the mold with the cross-linkable polyethylene material, resuming the rotation of the mold and heating it to a temperature sufficient to fuse and cross-link the polyethylene with the polyester layer, cooling the mold to a point below the melting point of the polyethylene and removing the finished structure from the mold.

United States Patent Vecchiotti 1 Aug. 19, 1975 1 1 HOLLOW,MULTl-LAYERED, 3.328.542 9/1970 Miller 206 2 CROSSJJNKEI) PLASTICSTRUCTURES H971 Nughcs 61/231 X 3,561,493 2/1971 Maillard 161/188 X ANDPROCESS FOR PRODUCING SAME Camillo W. Vecchiotti, Ridgewood, NJ.

Amer-ace Corporation. New York,

Oct. 29, 1973 Appl. No: 4l0,8l5

152] [1.5. Cl. 428/36; 136/181; 138/137; 138/146; 206/2; 220/DlG. 14;264/310; 428/188; 428/420; 428/482; 428/500 [51] Int. Cl.'-' B65D 85/84;B65D11/20; B32B 27/36; B32B 27/32 1581 Field of Search 220/83, 64, D10.14; 206/2; 161/188. 139, 231, 233, 247; 138/137, 146, DIG. 7

[56] References Cited UNITED STATES PATENTS 3,109,763 11/1963 Finger 161231 x 3,388,007 6 1968 Fiandt 206 2 X 3.391.823 7/1968 Tijms... 161/160x 3,408,234 10/1968 Ririeu 136/181 3,471,359 10 1969 Goldstein. 161/231x 3,491,799 1/1970 FOll 138 137 lrimur lixumii1e1'-Harold AnshcrAttorney, Agent, 01- FirmS, Michael Bender; R. A. Craig; Gregory J.Battersby l 1 ABSTRACT Hollow, multi-layered plastic structures areprovided comprising a layer of thermoplastic polyester material and alayer of polyethylene, the polyethylene and polyester layers beingcross-linked together at their interface. A process for producing suchstructures which comprises charging a mold with thermoplastic polyestermaterial, rotating and heating the mold so as to fuse and form a layerof the polyester material on the inner surface of the mold, cooling themold to a point below the melting point of the polyester but above themelting point of a subsequently added eross-linkable polyethylenematerial, stopping the rotation and charging the mold with thecross-linkable polyethylene material, resuming the rotation of the moldand heating it to a temperature sufficient to fuse and cross-link thepolyethylene with the polyester layer, cooling the mold to a point belowthe melting point of the polyethylene and removing the finishedstructure from the mold.

5 Claims, No Drawings 1 HOLLOW, MULTl-LAYERED, CROSS-LINKED PLASTICSTRUCTURES AND PROCESS FOR PRODUCING SAME BACKGROUND OF THE INVENTIONThis invention is directed to hollow, rotationally molded, multi-layeredplastic structures and -to a method for forming such structures. Moreparticularly, this invention relates to rotationally molded, hollowstructures formed from at least two plastic materials having varyingphysical and chemical properties, one of which is preferablycross-linkable.

The use of plastic in forming relatively large containers which areuseful for industrial applications, such as for example, containers forlarge storage batteries, is known in the art. However, because the useof certain plastics in forming such containers is economicallyimpractical, and because of the poor physical or chemical propertiesinherent in less expensive plastics, the use of such materials informing industrial scale structures has not been universally accepted.For example, cases used in manufacturing storage batteries which areinstalled in telephone exchanges are currently manufactured I fromthermoset compositions rather than thermoplastic materials. While such acomposition is said to have the desired chemical and load resistantproperties required for that particular use, cases formed from such hardrubber are quite heavy, are not flame resistant and have poor impactresistance.

Previous attempts to solve the apparent dilemma of balancing cost withrequired chemical and/or physical properties have included the formationof a multi-layer laminate of plastic materials, which are mechanicallyor adhesively bonded to each other, one layer having the desiredstructural strength properties and the other having resistivity toattack by chemicals. An example of such a multi-layered structure may befound in US. Pat. No. 3,391,823 to Tijms, which employs a layer offoamed polyethylene between a polyethylene layer and a polyester layerto effect a mechanical bond'therebetween. Another example of such astructure is seen in U.S. Pat. No. 3,652,368, to Forrno, which disclosesa method and a special apparatus for rotationally molding plasticmaterials in a manner which allows for a blendingof the materials attheir interface thus forming a mechanically cohesive bond therebetween.Other means of bonding dissimilar plastic materials together haverequired surface treatments of each plastic layer by methods includingflame, electrical discharge, chlorination, or chemical oxidation.

SUMMARY OF lNVENTION It has now been discovered that hollow,multi-layered plastic structures can be produced in which the respectivelayers are bound together at their interface by cross-linking of thepolymeric molecules. According to this invention, one of the layers is athermoplastic polyester, preferably a high melting polyester, while theother is formed from cross-linkable polyethylene. The process utilizedin forming such structures comprises charging a mold with a sufficientamount of polyester material to form a continuous coating of predetermined thickness on the inner surface of the mold, heating the mold to atemperature above thejmelting point of said polyester, rotating theheated mold forfasufficient period of time to fuse saidpolyester'andform a layer on the inner surface of the mold, cooling thero- LII tating mold to a temperature below the melting point of saidpolyester but above the melting point of crosslinkable polyethylene soas to crystallize said polyester layer, charging the slightly cooled butheated mold with a sufficient amount of cross-linkable polyethylene toform a continuous coating on said polyester layer, rotating said heatedmold for a sufficient period of time to form a polyethylene layer ofpredetermined thickness on said polyester layer and to effectcross-linking across said layers at their interface, cooling saidrotating mold to a point below the melting point of said polyethylenelayer so as to solidify the layers, and removing a hollow, multi-layeredstructure from the mold.

Accordingly, it is an object of this invention to produce a hollow,multi-layered plastic structure in which the respective layers arecross-linked together at their interface.

It is a further object of this invention to provide a hollow plasticstructure having an outer thermoplastic polyester layer and an inner,cross-linked thermoplastic polyethylene. layer.

It is still further an object of this invention to provide a process forproducing hollow, multi-layered plastic structures which arecharacterized by an outer layer having superior physical properties andan inner layer having resistivity to chemical degradation said layersbeing cross-linked together at their interface.

These and other objects and advantages of the presentinvention willbecome more apparent in the ensuing description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The hollow,multi-layered structure according to this invention comprises an outerpolyester layer and an inner polyethylene layer, both layers beingcross-linked together at their interface.

The polyester material utilized in forming these structures may beselected from any of the commercially available thermoplastic polyestersincluding linear polyester resins such as a linear, highly polymerizedester of terephthalic acid and a glycol of the series HO(CH )n OHwherein n is a whole number within the range of 2 to 10. An importantfeature of the selected polyester is its melting point which must behigher than that 'of the cross-linkable polyethylene utilized in thisinvention. In a preferred embodiment the polyester material will have amelting point between about 400 arid about 500F. Filler, includingfibrous materials such as glass, quartz, cotton, asbestos, ramie, andsisal, may be added to the polyester to improve its strength and otherproperties. In a preferred embodiment, the polyester will" be athermoplastic, glass-fiber filled, polyester based condensate polymermarketed by the Celanese Plastic Company under its mark Celanex.Although the particle size of the polyester is not critical to theinvention, in general, the polyester material should be finely dividedpowder and preferably, should have a particle size ranging from about 30to 40 mesh when passed through a standard sieve. l

The polyethylene material used in forming the second layer of theabove-indicated multi-layered structure must be cross-linkable with thepolyester of the first layer and accordingly, it must contain a cross-'linking agent; for example, a peroxide such as dicumyl peroxide,2,5-dimethyl 2,5-di (t-butyl peroxy) hexane,

or 2,5-dimethyl '-2,5-di (t-butyl peroxy) hexyne -3. In a preferredembodiment, the polyethylene will also contain a fire retardant agentsuch as, for example, any of the well known halogenated retardants. Thecrosslinkable polyethylene should be finely divided, for example a 50mesh powder and, preferably, have a melting point within the range250300F. A particularly preferred fire retardant, cross-linkablepolyethylene is marketed by U. S. 1. Chemicals, a Division of NationalDistillers and Chemical Corporation, under the mark Flamolin.

The production of the above described strueture according to thisinvention is accomplished in a rotational molding apparatus which isbiaxially rotated. Any conventional rotational casting equipment may beused for such purposes, such as, for example, any of the equipmentdescribed in the article by L. A. McKenna, Rotational Molding, ModernPlastics Encyclopedia, pages 780 et seq. (1967). Although the shape ofthe mold forms no part of the present invention, it will be appreciatedthat should it be desirable to produce molded shapes such as a batterycontainer, for example, the mold will generally have a correspondinghollow rectangular shape. Preferably, and in order to facilitate loadingof the mold and withdrawal of the molded article therefrom it isprovided at one end thereof with a detachable cover including agenerally smaller filling opening therein. Disposed in the filleropening in the mold cover is a removable plug or similar such closuremeans. The rate of r'otation of the mold is not critical to theinvention, but it should generally be of a sufficient rate to provide auniform distribution of the fused thermoplastic material on the innersurface of the mold. Speeds of 12 rpm on the major axis and 15 rpm onthe minor axis with a 4: 1 ratio of major to minor axis rotation havebeen found to be particularly effective in achieving such a uniformdistribution.

The process for producing the structures of this invention is carriedout by first removing the cover with its filling plug and charging themold with the desired amount of thermoplastic polyester. In the usualcase, the mold will be sprayed with a mold release agent prior to itscharging with the polyester which, in a preferred embodiment, will bepre-dried in a hot air circulating oven. After the mold is filled, thecover and its plug are replaced and the mold is passed into the heatingstage, usually an oven heated to a temperature between about 550F toabout 650F, preferably 600625F, and rotated biaxially in that stage fora sufficient period of time to fuse the polyester and form a uniformcoating in the mold. This is usually accomplished in about -30 minutes,preferably minutes. At the end of this period the mold is indexed to acooling chamber where biaxial rotation is maintained and the mold iscooled to a point below the melting point of the polyester but abovethat of the cross-linkable polyethylene. This cooling may beaccomplished with a fan and a cooling cycle of about 7l0 minutes,although, alternatively, other conventional heat exchange mediums may beused. At the end of the cooling cycle the mold temperature willgenerally be between about 280350F and the polyester layer on the innersurfaee thereof will be crystallized or set. At the end of this firstcooling cycle the rotational molding apparatus will be sto ped and withits cover maintained thereon the filler plug is removed to permit themold to be charged with the desired amount of cross-linkablepolyethylene through the plug filling'opening. Following the addition ofthe cross-linkable polyethylene to the mold and replacement of thefilling plug in the cover opening, the mold is indexed into an ovenstation set at a temperature between about 425-450F where it is rotatedbiaxially for a period between about 20-25 minutes. The speeds and ratioof rotation of the mold during this period will be the same as thoseused in the first heating stage. As the cross-linkable polyethylenecomes in contact with the crystallized or set surface of the polyesterit will fuse and become uniformly distributed over the surface of thepolyester layer by the centrifugal force of the rotating cycle. Bymaintaining the mold in the oven station at the above indicatedtemperature for the previously suggested period of time, crosslinkingbetween the polyester layer and the polyethylene layer will take placeat their interface. It will be noted that due to the temperature rangemaintained during this second oven stage there is no melting andconsequently, no dimensional distortion of the polyester layer on theinner surface of the mold. At the end of this oven cycle the mold willthen be indexed to a cooling chamber where it is cooled to a temperaturebelow the melting point of the polyethylene and to a point which permitshandling of the finished structure to remove it from the mold. This maybe accomplished, for example, by a cooling cycle which incliidcs the useof a fan for about 8 minutes followed by a water spray for about 10minutes and the fan again for about 2 minutes. When the mold has beenproperly cooled it is transferred to an unloading station where thecover is unclamped and removed from the mold to allow ready access tothe multi-layered molded structure within. This molded product isremoved from the mold and allowed to cool to room temperature in anupright position, usually overnight. in order to finish the resultingproduct, it may then be cut to its final desired dimension or form,e.g.. to produce a battery container-the top of the molded article iscut off to produce a container of the proper height.

. The following example will further illustrate a specific embodiment ofthe invention.

EXAMPLE A hollow, bi-layered, cross-linked plastic battery case wasproduced according to the following process. An electroformed, nickelmold having an end opening design and a removable Teflon filling plug inthe cover for the end opening was mounted on a McNeil hot air rotocastmachine No. l500-88 (three spindle unit which is capable of rotating themold biaxially. The mold had been cleaned previously and sprayed with acommercially available mold release agent (FRECOTE No. 33). 68 lbs. ofCELANEX No. 3310 (glass-fiber filled polyester resin) was charged to themold through its open end which was then closed and clamped tight. Thepolyester resin was in finely divided powder form having a particle sizeranging from about to about mesh. The charged mold was indexed into anoven set at 620F where it was allowed to rotate biaxially for 25 minutesat speeds of 12 rpm major axis and 15 rpm minor axis. These speedsproduced a 4:1 ratio of major to minor rotation on the machine. Thepolyester was thus fused and formed a uniform coating on the innersurface of the mold. At the end of this oven cycle the spindle ,wasindexed with the mold to a cooling chamber where the biaxial rotationwas maintained and a fan directed on the mold for 7 minutes to set thepolyester layer within. The spindle was then indexed with the mold to aload-unload station where rotation was stopped and the Teflon plugremoved from the filling opening in the mold end opening cover. Throughthis filler opening, lbs. of cross-linkable polyethylene (FLAMOLINMF-l57l l in finely divided powder form of about 50 mesh was dumped intothe mold and the filling plug replaced in the mold cover. Biaxialrotation was restarted and the spindle was then indexed with the mold toan oven station set at 450F where it was kept for minutes so that alayer of polyethylene was formed on and cross-linked to the polyesterlayer. The speeds and ratio of rotation thereof were the same asemployed in the first heating station. At the end of the second ovencycle the mold was positioned in a cooling chamber where it wassubjected to a fan for 8 minutes followed by a water spray for K)minutes and then a fan again for 2 minutes; thus lowering thetemperature to a point below the melting point of the polyethylene.Following this cooling, the mold was indexed to the unload station wherethe cover was unclamped from the mold and set aside. The mold was thentilted to a angle whereupon the molded battery case dropped out of themold with a slight pulling force on the case. After suitable cooling atroom temperature, the molded case was cut to desired height. Thepolyester layer in the finished product had a thickness of approximatelyone-fourth inch while the polyethylene layer was approximately 0.100inch thick.

As used in this description of the invention the term multi is intendedto encompass two layers of layered polymeric material.

The above described process may be used in the production or" a widevariety of structures including tanks, containers. vessels, barrels, andthe like. Particularly useful multi-layered structures producedaccording to this invention serve as cases for industrial sizebatteries. Such a case will have an outer surface with thecharacteristic structural strength and other properties of polyester butthe added feature of resistivity to chemical attack in its innersurface.

I claim:

1. A laminate comprising a first layer of a thermoplastic polyester anda second layer of a cross-linkable polyethylene, the melting point ofthe polyethylene beinglower than the melting point of the polyester,said layers being directly cross-linked at their interface.

2. A laminate as defined in claim 1 wherein said polyester contains afibrous filler material.

3. A laminate as defined in claim 1 wherein said polyester is a linear,highly polymerized ester of terephthalic acid and a glycol of the seriesHO(CH. ,)n OH wherein n is a whole number within thc, range of 2 to 10.

4. A laminate as defined in claim 1 in the form of a battery casecomprising a first layer of glass-filled thermoplastic polyester and asecond cross-linkable polyethylene layer, said layers being directlycross-linked at their interface. i

5. A laminate as defined in claim 1 wherein said polyethylene contains ahalogenated flame retardan't agent.

1. A LAMINATE COMPRISING A FIRST LAYER OF A THERMOPLASTIC POLYESTERA ANDA SECOND LAYER OF CROSS-LINKABLE POLYETHYLENE, THE MELTING POINT OF THEPOLYETHENE BEING LOWER THAN THE MELTING POINT OF THE POLYESTER, SAIDLAYER BEING DIRECTLY CROSSLINKED AT THEIR INTERFACE.
 2. A laminate asdefined in claim 1 wherein said polyester contains a fibrous fillermaterial.
 3. A laminate as defined in claim 1 wherein said polyester isa linear, highly polymerized ester of terephthalic acid and a glycol ofthe series HO(CH2)n OH wherein n is a whole number within the range of 2to
 10. 4. A laminate as defined in claim 1 in the form of a battery casecomprising a first layer of glass-filled thermoplastic polyester and asecond cross-linkable polyethylene layer, said layers being directlycross-linked at their interface.
 5. A laminate as defined in claim 1wherein said polyethylene contains a halogenated flame retardant agent.